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Ever see something so inefficient it evokes images of grape stomping to produce wine? I have, in Oracle 10g no less. A colleague of mine brought me a situation the other day that made me do a double-take, no triple-take, on a 10046 trace. The scenario involved a single row delete from a table using the index associated with the primary key on said table to delete the row, simple right? Well, the delete hung. The 10046 showed "db file sequential reads" spewing at a very rapid clip. The process was reading a child table that contained a column that referenced the primary key of the table being deleted. Okay, this is to be expected. We don't want to break our self-imposed business rules by orphaning child records. So what is my beef with this situation?
The child table had millions of rows that would have been orphaned had the delete succeeded. Keep in mind the constraint was NOT defined with ON DELETE CASCADE. Also, a single column index on the child table was associated with the child key. The stage was set for a swift and proper decline by Oracle to perform our delete. But this did not happen. Oracle was visiting ALL of the child rows then returning ORA-00292 "... - child record found." Yes, each and every child index entry was being visited. My colleague opened as SR with a very elegant little test case that reproduces the problem. Here it is. Try it for yourself and watch the trace with wonder and amazement. We have performed the test in 8i, 9i and 10g with the same results.
DROP TABLE CHILD;
DROP TABLE PARENT;
CREATE TABLE PARENT (COL1 NUMBER);
ALTER TABLE PARENT ADD CONSTRAINT PARENT_PK PRIMARY KEY (COL1);
CREATE TABLE CHILD (COL1 NUMBER);
CREATE INDEX CHILD_IX_01 ON CHILD (COL1);
ALTER TABLE CHILD ADD CONSTRAINT CHILD_FK_01 FOREIGN KEY (COL1) REFERENCES PARENT;
INSERT INTO PARENT VALUES (999999999999);
INSERT INTO CHILD VALUES (999999999999);
-- Insert approximately 1 million records into CHILD
for i in 1..20 loop
insert into child select * from child;
alter session set events '10046 trace name context forever, level 12';
DELETE FROM PARENT WHERE COL1 = 999999999999;
Why doesn't Oracle stop once it encounters the first index entry indicating a foreign key violation has just occurred? Isn't a single found entry sufficient to fail my statement? It seems a bit indulgent to check each and every child row irrespective of my barbaric attempt to break my own business rules. Is it a classic case of stupid is as stupid does? Nope. It is a good old fashioned Oracle bug.
By the way, the Oracle support analyst recommended putting the index associated with a child key in a read only tablespace as a workaround. Think about that for a second...